1. Field of the Invention
The present invention relates to a member that can be attached to the crown of a hydraulic machine. It also relates to a wheel equipped with such a member, and to a hydraulic machine comprising such a wheel.
2. Brief Description of the Related Art
In the field of hydraulic turbines, in particular Francis-type turbines, it is known to equip the part upstream of the crown of a wheel with an axisymmetric member, often termed a “tip”, the outer surface of which substantially extends the wet surface of the crown of the wheel, having a diameter which can reduce in a downstream direction. The purpose of this member or “tip” is to extend the guiding of the flow in the direction of the axis of rotation of the wheel. Similarly, it is known to use guide members or tips in turbine pumps and other turbines with blades. In the known hydraulic turbines, the flow traversing the wheel can give rise to vortices or turbulence, generally referred to as a “torche” and which, at certain loads, are corkscrew-shaped. This torche turbulence is problematic in that it causes pressure and/or power fluctuations that can adversely affect the stability of the power supply to the machine, as well as the mechanical performance of this machine.
WO-A-2005/038243 discloses the use of a tip comprising two surfaces which are respectively convergent and divergent in the direction of an axis of rotation of the wheel, which allows the turbulence to be limited to a large extent. However, turbulence persists at certain speeds.
U.S. Pat. No. 4,071,211 furthermore discloses the provision of curved blades on the outer surface of a wheel tip to assist the guiding of the flow when the load on a turbine is greater than the optimum operating conditions for which the wheel is designed. Such a solution is not capable of reducing or eliminating all the instabilities under low load and proves to be complicated to implement as it requires relatively elaborate analysis to define the complex geometry of the blades, and these blades are awkward to manufacture. Furthermore, the positioning of these blades on the outer surface of the tip means that they have a limited influence on the ropes which develop in the immediate vicinity of the axis of rotation of the wheel.
Moreover, GB-A-739 013 describes a Francis wheel equipped with a cone which bears fins arranged radially on the outside of the surface of the cone. At some speeds, these fins can disturb the course of a flow that travels over the outer surface of the cone.